The principles of distributed amplification have been known since a British patent application by Percival was first published describing the concept in 1937. W. S. Percival, "Improvements In and Relating to Thermionic Valve Circuits," British Patent Specification No. 460,562, 1936. Ginzton, et al. later described the first practical implementation of the concept in 1948, using vacuum tubes as the active devices. See "Distributed Amplification", Proc. I.R.E., V. 36, pp. 956-69, 1948. In essence, a distributed amplifier is an amplifier that produces an output by the successive gathering, along a uniform transmission line, of the contributions of multiple electronic amplifying devices.
Active electronic devices, those capable of amplification, contain spurious reactive elements, principally capacitance, associated with the physics of their operation and construction. A properly designed circuit employing these devices must contain appropriate compensation so that these reactances will have a minimal effect on the transfer of power within the desired operating frequency band. In conventional, not distributed, amplifying circuits this goal is achieved by resonating any residual reactance with a compensating reactance of the opposite sign, a procedure termed "conjugate matching." An amplifier designed on the basis of conjugate matching achieves the goal of optimum power transfer but with a substantial sacrifice in usable bandwidth.
A distributed amplifier design is realized by taking advantage of an alternative strategy for dealing with the spurious intrinsic device capacitances. When appropriately configured, a network containing both inductors and capacitors behaves as a short length of artificial transmission line over a very wide frequency band. Therefore, the input terminals of a plurality of electronic devices, containing extraneous intrinsic capacitance combined with appropriate compensating inductors, may be placed at regular intervals along an input transmission line while their output terminals are similarly placed along an output transmission line to achieve an aggregate network that retains very wideband performance.
In the original implementation and nearly all subsequent implementations the input and output transmission lines were designed to have a constant characteristic impedance throughout the network. Under this condition, the theoretical gain of the network becomes the sum of the gains of the individual contributing devices while the network bandwidth extends to the very high cutoff frequency beyond which the transmission line approximation of the spurious reactances no longer holds.
Although the conventional uniformly distributed amplifier is very attractive for applications requiring the highest gain-bandwidth product, it is not so well suited to applications requiring high output power. This is because the output power of a uniformly distributed amplifier is limited by the power handling capability of the final active device in the distribution. Each device contributes an equal output current onto the output transmission line. As these currents accumulate progressively in the direction of the load, the voltage must rise along the line in order that the ratio of the voltage-to-current, the characteristic impedance, may remain constant. This means that the voltage across each successive device in the cascade must inevitably be higher than its predecessor. Therefore, a uniformly distributed power amplifier must be designed such that the device nearest the load is operated within its rated voltage while all preceding devices remain suboptimally loaded. This limitation has been widely recognized for some time, K. B. Niclas, R. Pereira, and A. P. Chang, "On Power Distribution in Additive Amplifiers," IEEE Trans. Microwave Theory Tech., Vol. MTT-38, November 1990, and a variety of strategies have been proposed for increasing the power output of distributed amplifiers while maintaining their wide operating bandwidth.
One general strategy is to introduce some form of direct passive power combining into the circuit. A power amplifier realized by combining the outputs of two simultaneously driven distributed tiers of cells on a single output transmission line of uniform characteristic impedance was demonstrated in 1984. Y. Ayasli, L. D. Reynolds, R. L. Mozzi, and L. K. Hanes, "2-20 GHz GaAs Traveling-Wave Power Amplifier", IEEE Trans. Microwave Theory Tech., Vol. MTT-32, March 1984. While this approach does permit a factor-of-two power increase, it does not permit extracting the maximum power available from the cell combination. Also this technique is not extendible beyond two tiers.
Distributed power amplifiers have also been constructed with the active devices coupled to the input line through discrete series capacitors. B. Kim, and H. Q. Tserng, "0.5 W 2-21 GHz Monolithic GaAs Distributed Amplifier," Electronics Letters, 20, 288-289; and Y. Ayasli, S. W. Miller, and R. L. Mozzi, and L. K. Hanes, "Capacitively Coupled Traveling-Wave Power Amplifier," I.E.E.E. Transactions on Microwave Theory and Techniques, Vol. 32, 1704-1709. The series capacitors act with the intrinsic cell capacitance as a voltage divider to produce an impedance transformation that can be useful under certain circumstances. This approach, however, does not directly address the primary power constraint--the limitations in aggregate device power output.
Still a third approach to increasing power output is to increase the number of contributing cells by forming a higher order array. A two-dimensional array of device cells intended for power amplification was described in 1987 in which cascade connected one-dimensional input and output lines are intermeshed. See S. G. Houng, "2-D Distributed Amp Ups Power, Not Load," Microwaves & RF, April, 1987. Again, transmission lines having a uniform characteristic impedance along their length were proposed, resulting in a suboptimal power utilization of the plurality of device cells.
The possibility of changing the characteristic impedance of the output transmission line in order to increase the power output of a distributed amplifier has been recognized by others. A. G. Hughes and K. Wilson, "A novel approach to the design of a monolithic distributed power amplifier", IEE Colloquium on `Solid State Microwave Power Generation` (Digest No.88) p.6/1-5, 30 May 1986. The fundamental impedance conditions for optimum power output were not recognized which is evident from the use of a dummy load on the output line to compensate for reverse traveling waves.